1. Field of the Invention
The present invention relates to a so-called microwave Doherty amplifier, and more particularly, to a microwave Doherty amplifier which employs a way-extension method by which N ways (N≧2, N is a natural number) are established, and an envelope tracking method so that high efficiency and linearity of the power amplifier of a mobile communications base station or a handset can be achieved and price competitiveness and reliability of an apparatus employing the Doherty amplifier can be improved.
2. Description of the Related Art
As is well-known to those skilled in the art, a Doherty amplifier is a type of amplifier used in high efficiency modulation of a power transmitter, and improves efficiency by combining a class B amplifier, a class C amplifier, and an impedance inverse circuit. The Doherty amplifier employs a method connecting a carrier amplifier and a peak amplifier (or a peaking amplifier) in parallel using a quarter wave transformer (λ/4 line).
The peak amplifier (or peaking amplifier) of the Doherty amplifier controls the load impedance of the carrier amplifier by changing the amount of current provided to a load according to a power level so that efficiency improves.
The microwave Doherty amplifier was introduced by W. H. Doherty in 1936,and, in its initial stages, was used in an amplitude modulation (AM) transmitter of a broadcasting apparatus using a long frequency (LF) vacuum tube and a medium frequency (MF) vacuum tube.
Since the microwave Doherty amplifier was first applied to and used in an AM transmitter, a variety of suggestions for using the microwave Doherty amplifier in solid-state high output power transmitter have been made and many actual implementations have been proposed. One of the implementations is shown in FIG. 1, in which impedance is transformed by using quarter wave transformers 6 and 8 having characteristic impedances (Zm, Zb). In this circuit, only resistive matching by a matching unit 4 is possible. Reference number 2 indicates a splitter for distribution of the input. Za is a characteristic impedance of the line provided so that the two outputs of the splitter 2 have a phase difference of 90°.
The circuit shown in FIG. 2 is another embodiment of the prior art in which the microwave Doherty amplifier is applied to a solid-state high output power transmitter. By placing matching circuits 24 and 34 at the output parts of transistors (Q1, Q2) and placing offset lines 26 and 36 in latter parts of the matching circuits 24 and 34, this circuit enables matching of the imaginary part as well as the real part of the impedances so that the output power of the amplifier is maximized and Doherty operation is performed. Reference number 2′ indicates a splitter, reference number 20 indicates a carrier amplifying unit, reference number 30 indicates a peak amplifying unit, and Z1, Z2, and Z3 are characteristic impedances, respectively, of lines having the phase angles shown in FIG. 2.
However, though the Doherty amplifier increases efficiency when used in a high output power amplifier, it is insufficient to the improvement of linearity, which is needed for high capability and functionality of an apparatus.